Disclosed is a phosphor including at least one of a nitride and an oxynitride, wherein the nitride and the oxynitride contain an alkaline-earth metal element, silicon, and an activator element. wherein the phosphor has a volume average particle diameter of greater than or equal to about 50 nm and less than or equal to about 400 nm and an inner quantum efficiency of greater than or equal to about 60% at an excitation wavelength of about 450 nm. The method of preparing a phosphor includes a precursor preparation process of preparing a phosphor precursor particles including a silicon nitride particles, a compound containing an alkaline-earth metal element, and a compound containing an activator element, wherein the compound containing an alkaline-earth metal element and the compound containing an activator element are deposited on the surface of the silicon nitride particles; and a firing process of firing the phosphor precursor particles.